Risk tolerance
How much loss you can stomach without selling.
Risk tolerance is how much your investments can fall before you lose sleep, or worse, sell at the bottom.
Be honest with yourself
The best plan is one you will stick with. If watching a 40% drop would make you sell, a calmer mix you can hold through a crash beats a more aggressive one you abandon.
Looking at past drawdowns of a plan before you start is a good way to test your nerves in advance.